Gas separation is useful in many industries and can typically be accomplished by flowing a mixture of gases over an adsorbent material that preferentially adsorbs one or more gas components, while not adsorbing one or more other gas components. The non-adsorbed components are recovered as a separate product.
By way of example, one particular type of gas separation technology is swing adsorption, such as temperature swing adsorption (TSA), pressure swing adsorption (PSA), partial pressure purge swing adsorption (PPSA), rapid cycle pressure swing adsorption (RCPSA), rapid cycle partial pressure swing adsorption (RCPPSA), and not limited to but also combinations of the fore mentioned processes, such as pressure and temperature swing adsorption. As an example, PSA processes rely on the phenomenon of gases being more readily adsorbed within the pore structure or free volume of an adsorbent material when the gas is under pressure. That is, the higher the gas pressure, the greater the amount of readily-adsorbed gas adsorbed. When the pressure is reduced, the adsorbed component is released, or desorbed from the adsorbent material.
The swing adsorption processes (e.g., PSA and TSA) may be used to separate gases of a gas mixture because different gases tend to fill the micropore of the adsorbent material to different extents. For example, if a gas mixture, such as natural gas, is passed under pressure through a vessel containing an adsorbent material that is more selective towards water vapor than it is for methane, at least a portion of the water vapor is selectively adsorbed by the adsorbent material, and the gas exiting the vessel is enriched in methane. Before the adsorbent material reaches the end of its capacity to adsorb water vapor it is switched from an adsorption step to a regeneration step. Regeneration can be accomplished by raising the temperature of the adsorbent (TSA), purging the adsorbent with a dry stream (PPSA), reducing the pressure of the adsorbent (PSA) or by combinations of these methods. Once the adsorbent has been regenerated it is ready for another adsorption cycle. If a PSA step was used in the regeneration it has to be repressurized before it can be used in the next adsorption cycle.
Because natural gas produced from subsurface regions is typically saturated with water (H2O), dehydration is used to remove water to either pipeline specifications (e.g., in a range between 4 pounds per million cubic feet and 7 pounds per million cubic feet), NGL specifications (e.g., in a range between 0.1 parts per million (ppm) and 3 ppm), or LNG specifications (e.g., less than 0.1 ppm). Accordingly, typical methods and system utilize glycol dehydration along with an addition mole sieve dehydration system to remove water from a produced stream to provide a gaseous stream that satisfies specifications. The pipeline specifications may limit the water content to be less than about 4 pounds per million cubic feet to about 7 pounds per million cubic feet or the dew point has to be less than −5° F. to −15° F.
Similarly, for cryogenic processing conventional molecular sieve adsorbent beds are used to rigorous dehydrate the gas after glycol dehydration. The rigorous dehydration reduces water concentrations to less than 0.1 part per million (ppm) in a slow cycle TSA or PTSA process. The molecular sieve adsorbent beds are large because they are only regenerated once every hour to once a day. As such, the flow of regeneration gas out of the molecular sieve adsorbent bed is not steady and occurs in pulses when the molecular sieve adsorbent beds are regenerated. Further, the footprint of the slowly cycled molecular sieve adsorbent beds is large and the beds are heavy. The molecular sieve adsorbent beds typically use adsorbents, such as zeolite 5A and silica gel, which are prone to fouling. Moreover, adsorbent material in the molecular sieve adsorbent beds is configured as millimeter sized pellets that have mass transfer rate limitations in dehydration processes.
For example, U.S. Pat. No. 8,476,180 describes a process for regenerating a molecular sieve absorbent bed used for dehydrating an organic solvent. The process describes using the molecular sieve adsorbent bed for dehydrating ethanol, which includes a dehydrating cycle where an ethanol and water vapor mixture is loaded onto the molecular sieve adsorbent bed at a first temperature to absorb water and recover a substantially dehydrated ethanol vapor effluent. In a regeneration cycle, the molecular sieve adsorbent bed is subjected to a temperature swing technique whereby a dried gas, such as dried CO2, heated to a second temperature greater than the first temperature, is passed over the molecular sieve adsorbent bed. Water and residual ethanol are removed with the CO2 effluent and can be condensed and combined with a feed input for a subsequent dehydrating cycle. Unfortunately, this configuration relies upon the large slowly cycled heavy molecular sieve absorbent beds to handle the separation. Further, because of the long periods of time required to heat and regenerate such molecular sieve adsorbent beds, the molecular sieve units typically have a large footprint and are heavy.
As another example, Intl. Patent Application Publication No. WO2010/024643 describes a multi-tube type ethanol dehydration device that uses a pressure swing adsorption process in which producing dehydrated ethanol and regenerating an absorbent material are alternately performed in one multi-tube type bed. The dehydration device transfers heat by using a heat source generated during the absorption step. Again, the dehydration device as described uses long cycle times and has a larger footprint and are heavy.
As yet another example, U.S. Pat. No. 4,424,144 describes a method for shaping products of a 3A zeolite that are formed as beads or extrudates without any binder remaining. In this method, a 4A zeolite powder is mixed with a caustic solution and a metakaolin clay binder to form beads. Then, the beads are converted to a binderless 4A zeolite product, which is given a partial calcium exchange followed by a potassium exchange to obtain the desired 3A zeolite binderless bead. The size of the bead limits the mass transfer rate and the productivity. As a result, the rate at which feed is processed per unit of adsorbent material is significantly high.
Further, in addition to disadvantageous of certain types of configurations for dehydration, the intrinsic performance of the adsorbent material may be problematic. For example, in Lin et al., the fundamental adsorption kinetic data for water on single-layer 3A is given. The linear driving force coefficients are in the range between 3 per hour (h) and 7.4e-3/h (e.g., a range between 3/h and 7.4×10−3/h) for different partial pressures from 1.24 kPa and 3.1e-4 kPa (e.g., a range between 1.24 kPa and 3.1×10−4 kPa). See e.g., Lin et al., Kinetics of water vapor adsorption on single-layer molecular sieve 3A: experiments and modeling, IECR, 53, pp. 16015-16024 (2014). This process is slow as the kinetics are slow acting.
Further still, in Simo et al., a pilot scale adsorber apparatus was designed and constructed to investigate water and ethanol adsorption/desorption kinetics on 3A zeolite pellet for the design purposes of a fuel ethanol dehydration pressure swing adsorption (PSA) process. See, e.g., at Marian Simo, Siddharth Sivashanmugam, Christopher J. Brown, and Vladimir Hlavacek, Adsorption/Desorption of Water and Ethanol on 3A Zeolite in Near-Adiabatic Fixed Bed, Ind. Eng. Chem. Res., 48 (20), pp. 9247-9260 (2009). The breakthrough curves were utilized to study the effects of column pressure, temperature, flow rate, pellet size, and adsorbate concentration on the overall mass transfer resistance. The reference describes that the macropore and micropore diffusion mechanisms are the controlling diffusion mechanisms. The adsorbent is in pellet form with mass transfer resistances and rates.
Further, other publications describe the use of zeolite 4A in rapid cycle dehydration. These methods typically involve air drying and are not as fouling prone as treatment of natural gas streams. Indeed, many of the potential foulants in natural gas streams have the potential to diffuse into zeolite 4A over long exposure times. An example of the use of zeolite 4A in rapid cycle air drying is described in Gorbach et al. See Andreas B. Gorbach, Matthias Stegmaier and Gerhart Eigneberger, Compact Pressure Swing Adsorption Processes—Impact and Potential of New-type adsorbent-polymer monoliths, Adsorption, 11, pp. 515-520 (2005).
As another example, U.S. Pat. No. 4,769,053 describes a latent heat exchange media comprising a gas permeable matrix. The gas permeable matrix is formed of a sensible heat exchange material that is capable of absorbing sensible heat from a warm air stream and releasing the absorbed sensible heat into a cool air stream as the air streams flow through the heat exchange media. A layer of a coating composition comprising a molecular sieve is applied to at least a portion of the surface of the heat exchange material. The molecular sieve has pores that adsorbs moisture from a humid air stream flowing through the heat exchange media, and releases the adsorbed moisture into a dry air stream flowing through the heat exchange media. However, the heat exchange media does not appear to be capable of adsorbing contaminants from the respective streams.
While conventional approaches do perform dehydration on certain streams, these system have certain deficiencies, such as fouling and are not capable of handling rapid cycle processing of streams. Indeed, conventional systems, which may utilize adsorbent materials, such as 4A or 5A zeolites, silica or alumina, perform slow cycle dehydration processes. These processes involve equipment and units that have a larger footprint and/or weight more than rapid cycle processes.
Accordingly, there remains a need in the industry for apparatus, methods, and systems that provide enhancements in adsorbent materials for swing adsorption processes. Further, the present techniques provide adsorbent materials with enhanced kinetics for rapid cycle dehydration configurations, and enhanced foulant resistance. Accordingly, the present techniques overcome the drawbacks of conventional adsorbent materials.